Reference stable rails 4.0 release to avoid bundler issues

Workaround a bundler issue: bundler/bundler#2583

Error message:
```
Bundler could not find compatible versions for gem "actionpack":
  In Gemfile:
    rails (= 4.0.0) ruby depends on
      actionpack (= 4.0.0) ruby

    actionpack-action_caching (~> 1.0.0) ruby depends on
      actionpack (4.0.1.rc4)
```